The present invention relates to a process for the production of flexographic printing plates by engraving a printing relief into a laser-engravable flexographic printing element which has a photochemically crosslinked relief layer, where the relief layer is transparent and comprises an oxide, silicate or zeolite solid having a particle size of from 1 to 400 nm in an amount of from 0.1 to 8% by weight, based on the amount of all components of the relief layer.
In the technique of laser direct engraving for the production of flexographic printing plates, a relief which is suitable for printing is engraved directly into a relief layer which is suitable for this purpose. Although the engraving of rubber printing cylinders by means of lasers has in principle been known since the end of the 1960s, this technique has, however, only achieved broader commercial interest in recent years with the appearance of improved laser systems. The improvements in the laser systems include better focusability of the laser beam, higher power and computer-controlled beam guidance.
Laser direct engraving has a number of advantages over the conventional production of flexographic printing plates. A number of time-consuming process steps, such as the production of a photographic negative, and development and drying of the printing plate, can be omitted. Furthermore, the edge shape of the individual relief elements can be designed individually in the laser engraving technique. While the edges of a relief dot in photopolymer plates diverge continuously from the surface to the relief floor, laser engraving also enables the engraving of an edge which drops off vertically or almost vertically in the upper region and only spreads out in the lower region. Thus, at most slight dot gain, or none at all, takes place, even with increasing wear of the plate during the printing process. Further details on the technique of laser engraving are given, for example, in “Technik des Flexodrucks”, pp. 173 ff., 4th Edn., 1999, Coating Verlag, St. Gallen, Switzerland.
EP-B 640 043 and EP-B 640 044 disclose single-layered or multilayered elastomeric laser-engravable recording elements for the production of flexographic printing plates. The elements consist of “reinforced” elastomeric layers. The layer is produced using elastomeric binders, in particular thermoplastic elastomers, for example SBS, SIS or SEBS block copolymers. The reinforcement increases the mechanical strength of the layer. The reinforcement is achieved by means of certain fillers, by photochemical or thermochemical crosslinking or by combinations thereof. The job of the reinforcing fillers is to improve the mechanical properties of the laser-engravable recording elements, for example the tensile strength, rigidity or abrasiveness. Relatively large amounts of fillers are necessary for this purpose. The examples in EP-B 640 043 disclose the addition of from 10 to 25% by weight of carbon black, based on the sum of all components of the layer, as reinforcing filler.
Said recording materials may in addition also comprise strongly colored pigments or dyes as IR absorbers in order to increase the sensitivity to laser radiation. Carbon black has a double function and acts both as IR absorber and as reinforcing filler.
The use of strongly colored IR absorbers results in substantially opaque layers. Layers of this type can no longer be crosslinked photochemically as a whole, since the penetration depth of the actinic radiation is restricted owing to the very strong absorption. As a solution, EP-B 640 043 proposes producing a thick layer by casting a multiplicity of thin layers, in each case followed by photochemical crosslinking of each individual layer. However, this procedure is inconvenient, expensive and also makes other production plants necessary.
It is, however, in principle also possible to produce flexographic printing plates by laser engraving using commercially available photopolymerizable flexographic printing elements without IR absorbers. The sensitivity of conventional elastomeric binders to CO2 lasers (wavelength about 10 μm) is generally adequate for laser engraving. U.S. Pat. No. 5,259,311 discloses a process in which, in a first step, a conventional flexographic printing element is photochemically crosslinked by irradiation over the entire surface, and, in a second step, a printing relief is engraved by means of a laser.
The use of conventional flexographic printing elements for laser engraving has the major advantage that new production plants for a novel product line are not necessary, it being possible instead to use the existing plants.
However, a number of technical problems still remain to be solved in laser engraving of conventional flexographic printing elements.
In the ideal case, the relief layers of laser-engravable flexographic printing elements should not melt during the laser engraving, but instead direct transition of the degradation products into the gas phase should take place if at all possible. Prior melting of the layer is disadvantageous: melt edges may form around engraved recesses, and the edges of the relief elements become less sharp. Flexographic printing plates having irregularities of this type give prints of lower quality than do printing plates without such irregularities.
However, the relatively soft relief layers of conventional flexographic printing plates, in particular those comprising thermoplastic elastomers as binder, have a strong tendency to form melt edges during laser engraving.
Furthermore, the resolution of flexographic printing plates of this type is frequently unsatisfactory. In practice, the lines engraved by the laser are much broader than actually desired, with the consequence that two closely adjacent recesses which should in fact remain separate from one another through a center web coincide to give a single recess.